Optical glass

ABSTRACT

An optical glass suitable for precision press molding has a refractive index (nd) of 1.88 or over and an Abbe number (νd) within a range from 22 to 28, comprises, in mass %, 
     
       
         
               
               
               
             
                   
                   
               
                   
                 SiO 2   
                 15-25% 
               
                   
                 B 2 O 3   
                 0-5% 
               
                   
                 La 2 O 3   
                 0-5% 
               
                   
                 TiO 2   
                  5-15% 
               
                   
                 ZrO 2   
                  0-10% 
               
                   
                 Nb 2 O 5   
                 30-50% 
               
                   
                 WO 3   
                 0-5% 
               
                   
                 CaO 
                  0-10% 
               
                   
                 BaO 
                  0-10% 
               
                   
                 Li 2 O 
                  3-12% 
               
                   
                 Na 2 O 
                  0-10% 
               
                   
                 K 2 O 
                  0-10% 
               
                   
                 Bi 2 O 3   
                  0-15% 
               
                   
                   
               
           
              
             
             
              
              
              
              
              
              
              
              
              
              
              
              
              
              
             
          
         
       
     
     and has a glass transition point (Tg) within a range from 500° C. to 580° C. The optical glass preferably has a yield point (At) within a range from 550° C. to 640° C.

BACKGROUND OF THE INVENTION

This invention relates to an optical glass which has optical constantsof a refractive index (nd) of 1.88 or over and an Abbe number (νd)within a range from 22 to 28 and is suitable for precision pressmolding.

A typical optical glass having a high refractive index is one whichcontains a large quantity of lead oxide. Since this type of opticalglass is stable because of having high resistance to devitrification andhas a low glass transition point (Tg), it has been used for precisionpress molding. For example, Japanese Patent Application Laid-openPublication No. 1-308843 discloses an optical glass suited for precisionpress molding which comprises a large quantity of lead oxide.

Since, however, an environment in which precision press molding isperformed is kept in a reducing atmosphere for preventing oxidization ofthe mold, reduced lead precipitates from the surface of a glass if theglass contains lead oxide and this precipitating lead is deposited onthe surface of the mold with the result that difficulty arises inmaintaining the precision surface of the mold. Further, lead oxide isdetrimental to the environment and hence a lead-free optical glass hasbeen desired for.

Japanese Patent Application Laid-open Publication No. 52-45612 disclosesan optical glass of a SiO₂—Nb₂O₅—RO—R₂O system which is free of leadoxide. The refractive index (nd) of this optical glass, however, iswithin a range from 1.61 to 1.80.

Japanese Patent Application Laid-open Publication No. 2000-16830discloses an optical glass which does not contain lead oxide, has arefractive index (nd) of 1.7 or over and has a yield point of 580° C. orbelow. In this publication, however, there is no disclosure of aspecific example which has a refractive index (nd) of 1.88 or over.

It is an object of the present invention to eliminate the disadvantagesof the prior art optical glasses and provide an optical glass suited forprecision press molding which has a high refractive index (nd) of 1.88or over and an Abbe number (νd) within a range from 22 to 28 and alsohas a low glass transition point (Tg).

SUMMARY OF THE INVENTION

Laborious studies and experiments made by the inventors of the presentinvention for achieving the above described object of the invention haveresulted in the finding, which has led to the present invention, that anoptical glass having a glass transition point (Tg) within a range from500° C. to 580° C. and a yield point (At) within a range from 550° C. to640° C., being free of a substance which is detrimental to theenvironment, and has an excellent capability for precision press moldingcan be obtained in an optical glass having optical constants of arefractive index (nd) within a range from 1.88 to 1.95 and an Abbenumber within a range from 22 to 28.

For achieving the object of the invention, there is provided an opticalglass having a refractive index (nd) of 1.88 or over and an Abbe number(νd) within a range from 22 to 28, comprising, in mass %,

SiO₂ 15-25% B₂O₃ 0-5% La₂O₃ 0-5% TiO₂  5-15% ZrO₂  0-10% Nb₂O₆ 30-50%WO₃ 0-5% CaO  0-10% BaO  0-10% Li₂O  3-12% Na₂O  0-10% K₂O  0-10% Bi₂O₃ 0-15%

and having a glass transition point (Tg) within a range from 500° C. to580° C.

In one aspect of the invention, the optical glass has a yield point (At)within a range from 550° C. to 640° C.

The optical glass according to the invention has a refractive index (nd)of 1.88 or over and an Abbe number (νd) within a range from 22 to 28 andhas a low glass transition point (Tg) and therefore is suitable forprecision press molding. The optical glass preferably has a yield pointwithin a range from 550° C. to 640° C. The optical glass of the presentinvention has also excellent resistivity to devitrification and,therefore, is suitable for manufacturing a preform used for precisionpress molding.

DETAILED DESCRIPTION OF THE INVENTION

In the optical glass of the present invention, the above describedcontent ranges of the respective ingredients have been selected forreasons stated below. In the following description, the content rangesof the respective ingredients are expressed in mass percent. In thepresent specification, the phrase “be substantially free of” means thatthe ingredient is not intentionally added but may be mixed as anallowable amount of impurity.

The SiO₂ ingredient is an essential ingredient which works as a glassforming ingredient and is effective for improving resistance todevitrification. If the amount of this ingredient is less than 15%, thiseffect is insufficient whereas if the amount of this ingredient exceeds25%, the target refractive index (nd) of the present invention cannot beachieved. The amount of this ingredient therefore is limited to therange of 15-25%. A preferable range of this ingredient is 16-24% and amore preferable range thereof is 18-22%.

The GeO₂ ingredient is effective for improving resistance todevitrification and therefore may be added to the glass. Since, however,this ingredient is an expensive ingredient, a preferable range of thisingredient is 0-5%. A more preferable range of this ingredient is 0-4%and the most preferable range thereof is 0-2%.

The B₂O₃ ingredient is effective for improving resistance todevitrification and lowering the glass transition point (Tg). If theamount of this ingredient exceeds 5%, resistance to devitrificationdeteriorates rather than improves. The amount of this ingredienttherefore is limited to the range of 0-5%. A preferable range of thisingredient is 0-4% and a more preferable range thereof is 0.5-2%.

The La₂O₃ ingredient is effective for increasing a refractive indexwhile making the glass to one of a low dispersion characteristic. Thisingredient is also effective for improving resistance to devitrificationby coexisting with the B₂O₃ ingredient. If, however, the amount of thisingredient exceeds 5%, resistance to devitrification deteriorates ratherthan improves. The amount of this ingredient therefore is limited to therange of 0-5%. A preferable range of this ingredient is 0-4%.

The TiO₂ ingredient is an essential ingredient which is very importantfor increasing the refractive index (nd). If the amount of thisingredient is less than 5%, this effect is not satisfactory whereas ifthe amount of this ingredient exceeds 15%, resistance to devitrificationdeteriorates. The amount of this ingredient therefore is limited to therange of 5-15%. A preferable range of this ingredient is 8-14% and amore preferable range thereof is 10-13%.

The ZrO₂ ingredient is effective for increasing the refractive index(nd) and improving chemical durability of the glass. If the amount ofthis ingredient exceeds 10%, resistance to devitrification deteriorates.The amount of this ingredient therefore is limited to the range of0-10%. A preferable range of this ingredient is 0-8% and a morepreferable range thereof is 2-8%.

The Nb₂O₅ ingredient is an essential ingredient which is very importantfor satisfying the target refractive index of the present invention. Ifthe amount of this ingredient is less than 30%, this effect is notsufficient whereas if the amount of this ingredient exceeds 50%,resistance to devitrification deteriorates. The amount of thisingredient therefore is limited to the range of 30-50%. A preferablerange of this ingredient is one which exceeds 30% and is less than 50%and a more preferable range thereof is 32-48%.

The WO₃ ingredient is effective for increasing the refractive index (nd)and improving resistance to devitrification. If the amount of thisingredient exceeds 5%, resistance to devitrification deteriorates ratherthan improves. The amount of this ingredient therefore is limited to therange of 0-5%. A preferable range of this ingredient is 0-4% and a morepreferable range thereof is 0-3%.

The CaO ingredient is effective for improving resistance todevitrification and adjusting the optical constants. If the amount ofthis ingredient exceeds 10%, resistance to devitrification and chemicaldurability deteriorate. The amount of this ingredient therefore islimited to the range of 0-10%. A preferable range of this ingredient is0-8% and, more preferably, the glass should be substantially free ofthis ingredient.

The BaO ingredient is effective for improving resistance todevitrification and adjusting the optical constants. If the amount ofthis ingredient exceeds 10%, resistance to devitrification and chemicaldurability deteriorate The amount of this ingredient therefore islimited to the range of 0-10%. A preferable range of this ingredient is0-8% and a more preferable range thereof is 1-6%. For improvingresistance to devitrification and adjusting the optical constants, apreferable range of total amount of CaO and BaO is 0-12%, a morepreferable range of the total amount of these ingredients is 0.5-10% andthe most preferable range thereof is 1-8%.

The Li₂O ingredient is an essential ingredient which is effective forsubstantially lowering the glass transition point (Tg). If the amount ofthis ingredient is less than 3%, this effect is insufficient whereas ifthe amount of this ingredient exceeds 12%, resistance to devitrificationdeteriorates sharply. The amount of this ingredient therefore is limitedto the range of 3-12%. A preferable range of this ingredient is 4-11%and a more preferable range thereof is 5-10%.

The Na₂O ingredient is effective for improving chemical durability bythe mixed alkali effect while lowering the glass transition point (Tg).If the amount of this ingredient exceeds 10%, resistance todevitrification deteriorates. The amount of this ingredient therefore islimited to the range of 0-10%. A preferable range of this ingredient is0-8% and a more preferable range thereof is 0-6%.

The K₂O ingredient is effective for improving chemical durability by themixed alkali effect while lowering the glass transition point (Tg). Ifthe amount of this ingredient exceeds 10%, resistance to devitrificationdeteriorates. The amount of this ingredient therefore is limited to therange of 0-10%. A preferable range of this ingredient is 0-8% and a morepreferable range thereof is 0-6%. A preferable range of the total amountof the Li₂O, Na₂O and K₂O ingredients is 3-32%, a more preferable rangethereof is 4-24% and the most preferable range thereof is 6-16%.

The Bi₂O₃ ingredient is effective for lowering the glass transitionpoint (Tg) while maintaining a high refractive index (nd). If the amountof this ingredient exceeds 15%, resistance to devitrificationdeteriorates. The amount of this ingredient therefore is limited to therange of 0-15%. A preferable range of this ingredient is 0-12% and, morepreferably, the glass should be substantially free of this ingredient.

The Sb₂O₃ ingredients may be optionally added as a refining agent duringmelting of the glass materials. Addition of this ingredient up to 1%will suffice.

The optical glass of the present invention should preferably be free ofPbO and F₂ which are not suitable for an optical glass for precisionpress molding.

EXAMPLES

Examples of the optical glass of the present invention will be describedbelow. The scope of the present invention however is not limited bythese examples.

The following Tables 1 to 3 show compositions of Examples Nos. 1-13 ofthe optical glass of the invention as well as refractive index (nd),Abbe number (νd), glass transition point (Tg) and yield point (At) ofthese examples.

For manufacturing the optical glasses of Examples Nos. 1-13,conventional raw materials such as oxides, carbonates and nitrates wereweighed and mixed at a predetermined ratio to constitute thecompositions shown in the tables. These raw materials were put in aplatinum crucible, melted at a temperature within a range from 1100° C.to 1300° C. for two to five hours and stirred and homogenized. Afterlowering the temperature to a proper temperature, the melt was cast intoa mold and annealed. As to Examples Nos. 1-13, colorless and transparentglasses were obtained.

TABLE 1 Examples 1 2 3 4 5 SiO₂ 20.0 21.0 21.0 24.0 18.0 B₂O₃ 1.5 2.02.0 0.0 5.0 La₂O₃ 0.0 3.0 3.0 0.0 4.0 TiO₂ 12.0 12.0 12.0 15.0 10.0 ZrO₂6.0 6.0 6.0 8.0 6.0 Nb₂O₅ 40.4 38.9 38.9 34.0 39.9 WO₃ 0.0 2.0 2.0 0.00.0 CaO 4.0 2.0 2.0 1.0 0.0 BaO 4.0 4.0 4.0 6.0 8.0 Li₂O 9.0 9.0 6.010.0 9.0 Na₂O 0.0 0.0 0.0 0.0 0.0 K₂O 0.0 0.0 3.0 0.0 0.0 Sb₂O₃ 0.1 0.10.1 0.0 0.1 GeO₂ 3.0 0.0 0.0 2.0 0.0 Total 100.0 100.0 100.0 100.0 100.0nd 1.9011 1.9011 1.8973 1.9091 1.9191 νd 25.2 25.2 24.4 25.9 24.0 Tg 539540 552 542 521 At 589 589 601 572 572

TABLE 2 Examples 6 7 8 SiO₂ 21.0 21.0 25.0 B₂O₃ 4.0 0.5 0.0 La₂O₃ 3.03.0 0.0 TiO₂ 13.0 14.0 8.0 ZrO₂ 5.0 6.0 2.0 Nb₂O₅ 36.0 38.5 50.0 WO₃ 3.04.0 0.0 CaO 0.0 0.0 0.0 BaO 4.0 2.0 2.0 Li₂O 4.0 5.0 5.0 Na₂O 6.0 0.00.0 K₂O 0.0 6.0 8.0 Sb₂O₃ 1.0 0.0 0.0 GeO₂ 0.0 0.0 0.0 Total 100.0 100.0100.0 nd 1.9069 1.9289 1.8838 νd 23.8 24.2 26.6 Tg 523 564 565 At 551633 592

TABLE 3 Examples 9 10 11 12 SiO₂ 21.0 21.0 20.0 21.0 B₂O₃ 2.0 2.0 1.92.0 La₂O₃ 3.0 3.0 3.0 3.0 TiO₂ 12.0 12.0 12.0 12.0 ZrO₂ 6.0 6.0 6.0 6.0Nb₂O₅ 38.9 30.4 30.0 38.9 WO₃ 2.0 2.0 2.0 2.0 CaO 0.0 2.0 2.0 0.0 BaO4.0 4.0 4.0 4.0 Li₂O 4.0 9.0 9.0 9.0 Na₂O 4.0 0.0 0.0 2.0 K₂O 3.0 0.00.0 0.0 Sb₂O₃ 0.1 0.1 0.1 0.1 GeO₂ 0.0 0.0 0.0 0.0 Bi₂O₃ 0.0 8.5 10.00.0 Total 100.0 100.0 100.0 100.0 nd 1.8875 1.8965 1.9048 1.8905 νd 23.925.1 24.8 25.1 Tg 545 519 520 519 At 597 568 570 568

The glass transition point (Tg) and yield point (At) were obtained froma thermal expansion curve which was obtained by heating a specimenhaving a length of 50 mm and a diameter of 4 mm by elevating temperatureat a constant rate of 4° C. per minute and measuring elongation of thespecimen and temperature.

As described above, the optical glass of the present invention hasoptical constants of a refractive index (nd) of 1.88 or over and an Abbenumber (νd) within a range from 22 to 28 and has a glass transitionpoint (Tg) within a range from 500° C. to 580° C. and, therefore, it issuitable for precision press molding. Since the optical glass hasexcellent resistance to devitrification, it is suitable formanufacturing a preform for precision press molding of the droppingmethod.

The optical glass of the present invention is suitable for use as a lensfor optical communication for which demand is rapidly increasing inrecent years. The lens for optical communication is a glass lens whichhas functions such as coupling laser beam emitted from an illuminantsuch as a semiconductor laser to an optical fiber with high efficiencyand, therefore, a micro-optical part which is indispensable for opticalcommunication. For this lens, a ball glass or an asphexical lens isnormally used and this lens is required to have a high refractive index.The optical glass of the present invention is suited for precision pressmolding for producing an aspherical lens.

What is claimed is:
 1. An optical glass having a refractive index of(nd) of 1.88 or over and an Abbe number (νd) within a range from 22 to28, comprising, in mass %, SiO₂ 15-25% B₂O₃ 0-5% La₂O₃ 0-5% TiO₂  5-15%ZrO₂  0-10% Nb₂O₅ more than 30 and less than 50% WO₃ 0-5% CaO  0-10% BaO 0-10% Li₂O  3-12% Na₂O  0-10% K₂O  0-10% Bi₂O₃  0-15%

and having a glass transition point (Tg) within a range from 500° C. to580° C.
 2. An optical glass as defined in claim 1 having a yield point(At) within a range from 550° C. to 640° C.